Many access networks, in particular passive optical networks (PONs), provide a range of broadband and narrowband services using two-way communications between an access node (AN) and network terminals (NTs). PON is a widely used network architecture for residential and business broadband access. PONs are considered to be inexpensive for network operators because they do not require any active equipment or power supplies between the operator's central office (CO) and customer's premises (CP). In a typical PON an optical line termination (OLT) device provides downstream communications to multiple NTs, termed optical network units (ONU)s or optical network termination (ONT) devices. Typically, the downstream frames sent contain information for multiple NTs.
In the ITU-T and IEEE standards, such as ITU-T recommendation G.984.3, “Gigabit-capable Passive Optical Networks (GPON): Transmission convergence layer specification, March 2008 and IEEE P802.3av-D3.3, “Physical layer specifications and management parameters for 10 Gb/s passive optical networks”, IEEE 802.3 amendment, May 12, 2009, both of which are incorporated herein in their entirety by reference, the downstream is either not protected or a forward error correction (FEC) code may be used. Forward error correction (FEC) is often used in communication systems, and is based on transmitting the data in an encoded format. The encoding introduces redundancy, which allows the decoder to detect and correct transmission errors.
Typically, a systematic code such as a Reed Solomon (RS) code is used. A systematic code is a code where the part that carries the information is not transformed. The check symbols are computed and appended. If one does not use a decoder, it is sufficient to drop the check symbols. No other operation is required to be performed to retrieve the data (this is unlike, for instance, many convolutional codes), which is seen as one of the advantages of using systematic codes. Another advantage is the burst error correcting capability. Since the code corrects “symbols” of m bits each, if multiple consecutive bits are highly likely to be in error (a burst error), then it “counts” as only a few symbol errors.
The main motivation for using the FEC is the ability to operate at a lower signal to noise ratio (SNR) and its associated (higher) input bit error rate (BERi) while maintaining a low output bit error rate (BERo). For example, the (255,239) RS code described above provides a BERo below 10−15 for a BERi up to 10−4. This translates to an increase of the link budget of approximately 3-4 dB, and a reduction in data rate of approximately 7% when compared to no FEC. It should be noted that the OLT can disable the FEC, in which case the frames are sent without FEC protection.
The links between the OLT and the different ONUs typically have different SNRs and corresponding different BERis. If the link between the OLT and an ONU is good such that the BERi is already low, strong FEC is undesirable, because such FEC would require redundancy levels that reduce the transmission rate. At the same time, for a link between the OLT and an ONU that has a low SNR and a correspondingly high BERi, the standard level of FEC may not be sufficient to provide an acceptable BERo. As such, for a system with an OLT and multiple ONUs, the performance in terms of downstream transmission rate for a specified maximum BERo is determined by the worst OLT-ONU link.
Thus, what is required is a system and method for providing improved transmissions from an OLT to ONUs in a PON network.